Microneedles are considered a hybrid of hypodermic needles and transdermal patches that involve narrow and shallow injections to administer drugs. Microneedles are a new drug delivery system for transdermal delivery of drug, by virtue of their small dimensions, ability to penetrate into skin tissue in a non-invasive manner and their minimization of the risk for infection. Microneedles typically are around 100-200 microns in length, which is long enough to penetrate the stratum corneum skin barrier but not deep enough to reach the nerves in the dermis layer. Therefore, microneedles may provide a painless injection and may allow patients to operate the microneedles safely on their own.
Microneedles are conventionally made of silicon, glass, metallic material or polymer. Early microneedles were made of silicon and were prepared by microlithography and etching technologies originally developed for the microelectronics industry. Glass microneedles are made using conventional drawn glass micropipette techniques. However, the fragile nature of silicon and glass often results in easy breakage of the microneedles.
Metallic materials, including steel, stainless steel and nickel, may be electrodeposited onto polymeric mold to create microneedles with good mechanical strength. However, the possible toxicity of metals to the human body can limit its area of application and can pose a problem for the waste disposal. Polymeric microneedles are easy to manufacture, safe and inexpensive. A number of biodegradable polymers have been approved by the U.S Food and Drug Administrative (FDA). For example, Poly(lactic-acid) (PLA), poly(glycolic-acid) (PGA) and their copolymers have been used routinely for a narrow range of medical applications. However, the relative low Young's modulus of polymers means the polymer microneedles may not be strong enough to penetrate the skin.
Consequently, it is desirable to develop microneedles with improved properties. Particularly, it is desirable to develop microneedles that exhibit improved mechanical properties and possess improved thermal and chemical stability compared to silicon and polymer microneedles. It is also desirable to develop microneedles that are biocompatible, especially when compared to metal microneedles. Furthermore, it is desirable to develop microneedles that can be produced conveniently and inexpensively.